Using a finite-element mode solver, the modes of a commercial endlessly single-mode photonic crystal fiber (ESM-PCF) were investigated. Based on the loss discrimination between the dominant and the nearest higher order mode, we set-up a criterion for the single-modeness. Using that measure, we verified the single-modeness of the corresponding ESM-PCF and found that the endlessly single-modeness are valid only for relatively long fiber-length applications. Based on this, we suggest that ESM-PCF for short fiber-length applications should be designed differently from those for long fiber-length applications.